Gemini Near-Infrared Integral-Field Spectrograph (NIFS)

System Engineering

10.1 Instrument Configuration Management

10.1.1 Environmental Heat Load

The maximum acceptable heat load that can be conducted into the instrument body is 50 W and the maximum acceptable heat load that can be convected into the dome is 50 W. The heat generated by the closed-cycle coolers is excluded from this budget. The majority of the heat generating electronic components will be mounted inside the two cooled electronics enclosures. The two items that need to be mounted on the cryostat are the OIWFS SDSU-2 detector controller and the spectrograph SDSU-2 detector controller. The OIWFS detector controller generates ~ 45 W and will be heat sunk to the instrument body as with NIRI. The spectrograph detector controller and its power supply generate ~ 95 W and will be water cooled as discussed in §8.3.8.

10.1.2 Weight Budget

The preliminary weight budget for NIFS is presented in Table 1.

Table 1: Preliminary NIFS Weight Budget

Item	Mass (kg)	Source

ISS Interface	187	AutoCAD solid model
Main Frame (and partial ballast)	500	Estimate
Vacuum Jacket	315	Young et al. (1998)
OIWFS	147	Young et al. (1998); includes cold work surface
Spectrograph	40	Estimate
Detector Controllers (2 of)	10	Estimate
Thermal Enclosures (2 of)	330	From Gemini ICD
Detector Controller Power Supplies (2 of)	15	Estimate
IOC (2 of)	40	Estimate
ICS Stepper Motor Drive Frame	20	Estimate
ICS Sensor Interface Frame	10	Estimate
ICS Temperature Controllers (3 of)	15	Estimate
ICS Temperature Control Hardware	20	Estimate
Cabling and Hoses	50	Estimate
Miscellaneous	40	Estimate

Subtotal	1739
Ballast	261

Total	2000

10.1.3 Space Restrictions

NIFS will occupy the same space envelope as NIRI.

10.1.4 Electronics Cabinet Space Allocation

There will be two thermal enclosures. The mechanism controller, temperature controller, their IOC, and the OIWFS detector controller power supply will be located in one thermal enclosure. The science detector controller VME interface card, and the detector controller IOC will be located in the other thermal enclosure. This duplicates the arrangement for NIRI.

10.2 Differential Atmospheric Refraction

Performing imaging spectroscopy at high spatial resolution requires consideration of the effects of differential atmospheric refraction. Atmospheric refraction shifts the apparent positions of objects by an amount dependent on the zenith distance of the object and the wavelength of observation. Differential atmospheric refraction will affect NIFS in two ways; light at one end of a recorded spectrum will originate from a slightly different position on the sky to light at the other end of the spectrum, and the apparent separation and orientation between a science object and its OIWFS guide star will vary with time during long observations.

Similar considerations apply to both NIFS and GNIRS. The analysis presented here follows the analysis of differential atmospheric refraction effects in GNIRS (GNIRS SDN0019 “Differential Refraction”).

10.2.1 Differential Atmospheric Refraction Model

The refractive index of air at temperature T = 15°C, pressure P = 760 mm Hg, and at a vacuum wavelength of l_vac (mm) is given by

where s = l_vac^-1 (Edlén 1953). The refractive index is adjusted to temperatures and pressures appropriate to Mauna Kea (T = 0°C and P = 452 mm Hg) using the expression

(Barrell & Sears 1939). The Earth’s atmosphere can be modeled as a flat slab of air with uniform thickness and refractive index n_T,P. Atmospheric refraction causes an angular displacement, z, of the apparent zenith angle of an object, z_app, from its true zenith angle, z_true, which is given by

The differential refraction between two closely spaced objects is then obtained by differentiation such that

where dz is the true zenith angle difference between the two objects and the constant is applicable over the wavelength range 0.9-2.5 mm.

10.2.2 Implications for Guiding

NIFS will use the OIWFS to offset guide the science object with OIWFS guide stars located up to 60² from the science object (with ALTAIR). The OIWFS will normally be operated in the same wavelength range as the science observation by appropriate selection of the OIWFS filter. As was found for GNIRS, differential refraction between the science object and the OIWFS guide star can then amount to ~ 0.066² (~ 1.6 pixels) over 4 hr integrations in which the zenith distance changes by ~ 60°. This offset will be corrected by the Telescope Control System (TCS) offsetting the OIWFS X-Y gimbal position during the exposure. A wavelength-dependent offset will have to be applied if the OIWFS is not operated at the same wavelength as the science instrument.

10.2.3 Implications for Science Observations

Differential refraction also causes the image of an object to appear at different positions in the telescope focal plane depending on the wavelength of observation. If the NIFS IFU slitlets are aligned parallel to the vertical direction, the red and blue ends of an object spectrum will appear at different spatial positions in the slitlet. These displacements are shown as functions of zenith angle for the NIFS option A gratings (§4.6.2.1) in Figure 1 to Figure 4. The effect is significant above a zenith angle of ~ 60° for the high resolution J1 and J2 gratings and the low resolution J and HK gratings. The ALTAIR atmospheric dispersion corrector should be used for observations with these gratings. This will introduce a further emissivity penalty to K band observations with the HK grating. The option B gratings (§4.6.2.2) have wider wavelength coverage, and so will suffer larger atmospheric dispersion effects.

Figure 1: Differental atmospheric refraction between extreme wavelengths recorded with the high resolution J1 grating.

Figure 2: Differental atmospheric refraction between extreme wavelengths recorded with the high resolution J2 grating.

Figure 3: Differental atmospheric refraction between extreme wavelengths recorded with the high resolution H grating.

Figure 4: Differental atmospheric refraction between extreme wavelengths recorded with the high resolution K grating.

Figure 5: Differental atmospheric refraction between extreme wavelengths recorded with the low resolution J grating.

Figure 6: Differental atmospheric refraction between extreme wavelengths recorded with the low resolution HK grating.

10.3 Calibration Requirements

NIFS data frames will require bias subtraction, dark frame subtraction, flatfielding, wavelength calibration, correction for geometrical distortion, flux calibration, and correction for absorption in the Earth’s atmosphere. A related, but different, calibration problem will be to determine the PSF appropriate to each science observation. NIFS will use continuum and emission-line lamps in the Gemini Calibration Unit (GCAL) for flatfield and wavelength calibration, respectively, using the near-infrared diffuser in GCAL. The lamp intensities have been specified for NIRI and GNIRS. The pixel scale and spectral resolution of NIFS are similar to modes of GNIRS, so the GCAL lamp intensities should also suit NIFS.

10.3.1 Bias Frames

Bias frames are needed to determine the DC electrical offsets for each detector pixel. NIFS will include a blanked off position in the filter wheel that will exclude external light from the remainder of the optical train. Bias frames will be recorded with this blank in place using the minimum possible exposure time (~ 5 s). Zero length exposures are not possible due to the readout architecture.

10.3.2 Dark Frames

Dark frames are used to remove the signal component due to spontaneously generated charge and background light from within the cryostat that does not scale in the same way as light entering from outside the cryostat. Dark frames will be recorded using the filter wheel blank as for bias frames.

10.3.3 Flatfield Frames

Throughput variations and pixel-to-pixel quantum efficiency and gain variations cause pixel-to-pixel signal variations in response to uniform illumination. NIFS will use continuum lamps in GCAL for flatfield calibration. The beam from GCAL is injected below ALTAIR, so flatfield frames obtained in this way do not allow for throughput variations within ALTAIR. This should not be a problem given the small field-of-view of NIFS. Spectroscopic flatfield frames are strictly needed only for the removal of pixel-to-pixel sensitivity variations. Large scale sensitivity variations in the spectral direction are calibrated using measurements of flux standard stars. Large scale sensitivity variations in the spatial direction can be calibrated using measurements of twilight sky spectra.

10.3.4 Wavelength Calibration Frames

Arc lamp spectra recorded during daylight will provide the primary transformation of NIFS spectra to known linear wavelength scales. NIFS will use the near-infrared arc lamps in GCAL as its wavelength calibration source. The required range of lamps and intensities will be similar to those needed for modes of GNIRS. The need to move M3 in the Instrument Support Structure to inject the GCAL beam could potentially compromise the positional repeatability of NIFS science observations. If this proves to be problematic, it will be possible to use sky spectra containing OH airglow emission to track wavelength zero point shifts during a night.

10.3.5 Geometric Distortion

NIFS will form a reformatted slit image that will be stepped because of the image slicing function of the IFU, and be distorted by spectral line curvature induced by the grating as well as other optical aberrations. Geometrical distortions in the spectral direction will be calibrated using arc lamp exposures as part of the wavelength calibration. Geometrical distortions in the spatial direction must be treated separately. It is not feasible to record stellar spectra at several positions along each slitlet in a manner analogous to long-slit spectroscopy because of the time involved and the need to perform this calibration during daytime. NIFS will use a multislit mask in the Focal Plane Mask Wheel in combination with the flatfield lamp in GCAL to produce multiple artificial star spectra that will be used to calibrate spatial distortions. The multislit mask will be oriented perpendicular to the IFU slitlets so it will illuminate an array of points along all of the IFU slitlets simultaneously. One central artificial star would be sufficient to trace the curvature (i.e., offset) of each IFU spectrum on the detector. Two artificial stars measure offset and linear stretch. More artificial stars allow higher order distortions to be measured, although we expect that offset and linear stretch will be sufficient. The NIFS field-of-view is ~ 2´2 mm at the Focal Plane Mask Wheel. NIFS will use a Ronchi grating with 200 mm pitch rulings which will produce 10 artificial star images of ~ 100 mm (~ 0.16² º 4 pixels) width along each IFU slitlet image.

10.3.6 Flux Calibration

Measurements of stellar flux standards will be used for absolute flux calibration of NIFS spectra. Flux standards should be measured close in time and position on the sky to science observations to minimize effects due to atmospheric transparency variations. Suitable flux standards will be drawn from lists of photometric standard stars.

10.3.7 Terrestrial Absorption

Near-infrared spectra suffer strong, time-variable, wavelength-dependent absorption in the Earth’s atmosphere. Systematic errors in correcting for this absorption can dominate statistical errors, and are likely to be particularly significant for the long (~ 1 hr) exposures required to overcome dark current and read-noise with the high resolution, short wavelength NIFS gratings. Smooth spectrum stars must be measured close in time and position on the sky to science observations to achieve accurate correction. Dwarf stars of F and G spectral type have the weakest spectral features and are the most common and widely distributed smooth spectrum stars over the sky. Intrinsic absorption in the smooth spectrum, solar-analog stars can be modeled in the manner described by Maiolino, Rieke, & Rieke (1996). Smooth spectrum stars are commonly drawn from the Bright Star Catalog. Most of these stars are brighter than the K ~ 6.4 mag limit for measurement with NIFS (§3.2.1). It will therefore be necessary to define alternative lists of smooth spectrum, solar-analog stars for NIFS and, presumably, also for GNIRS.

10.3.8 Atmospheric Dispersion Effects

Atmospheric dispersion will produce a wavelength dependent position offset on the NIFS science detector. This effect has been considered in §10.2 and found to be small. If deemed important, the IFU slitlets can be oriented at the mean parallactic angle so that atmospheric dispersion smears images along slitlets. By modeling the effects of atmospheric dispersion, the spatial map for each slitlet in the detector plane can be adjusted as a function of wavelength so that the same region of sky is extracted at all wavelengths. Such observations should be performed away from the Zenith to minimize field rotation.

10.3.9 Non-Common Path Phase Errors

Phases errors generated in the non-common path between ALTAIR and NIFS will be determined so that ALTAIR can apply appropriate compensation to the corrected wavefront. The non-common path phase errors will be recorded during daylight by recording in-focus and out-of-focus images of an artificial star generated by ALTAIR. It is proposed that NIFS will not have a detector focus stage. ALTAIR is able to defocus the star image. Vignetting of the defocussed beams caused by the NIFS IFU requires further investigation.

10.4 Handling Equipment

The entire NIFS instrument will be integrated and tested in the RSAA workshop assembly-room at Mt Stromlo Observatory. The assembly area floor space measures ~ 5000´4600 mm and has a ceiling clearance to an overhead I-beam of ~ 4600 mm. The I-beam is suitable for lifting a 3 tonne mass but will require Occupational Health & Safety clearance before twin chain hoists can be fitted. Access to the room is via high double doors that are 1600 mm wide. This is wide enough to pass all NIFS components including the carrier frame. The overhead I-beam is high enough to allow full tilt tests to be performed on the assembled instrument. The electronics rack carrier supports and electronics racks will be added to the carrier frame once it is in the room.

10.4.1 Carrier Frame Air Pallet and Beam

RSAA will either build a simplified version of an instrument air pallet for moving the instrument, or source one for the assembly period from Gemini. The carrier pallet will measure ~ 1400´1400´300 mm and have Gemini standard instrument interface pads fitted to its top surface. Air pads will be fitted to the corners of the pallet as suitable wheels with 1 tonne capacity are not readily available. The Gemini air pallet layout is described in ICD 1.9/2.7.

A suitable 2 tonne lifting beam similar to that shown in ICD 1.9/2.7 will either be built by RSAA for tilt testing the assembled instrument, or sourced from Gemini.

10.4.2 Clean Room Trolleys, Hoist, and Beam

The NIFS optical system and vacuum jacket will be assembled in a clean room environment using anti-static precautions. A 150 kg chain hoist will be fitted to the ceiling of the RSAA clean room and a small, purpose-built lifting beam will be manufactured to lift NIFS sections during assembly.

Two flatbed trolleys will be purchased to transport the vacuum jacket and spectrograph sections to and from the clean room. These two trolleys can be any suitably-sized commercially-available units having a 500 kg load capacity and measuring ~ 1000 mm square.

10.5 Risk Summary

Specific risks associated with each aspect of the NIFS project have been identified in each section. We summarize the most significant risk areas in Table 2.

Table 2: NIFS Risk Mitigation Strategy

Item	Area of Concern	Science Impact	Risk Level	Risk Mitigation Plan
1	Science
	- Sensitivity	HIGH	MEDIUM	Model performance, use lowest dark current detector, and control scattered light.
	- Guide star availability	MEDIUM	MEDIUM	Identify typical objects with suitable guide stars and control flexure.
	- PSF determination	MEDIUM	MEDIUM	ALTAIR group developing AO control loop model.
2	Optical
	- IFU	HIGH	HIGH	Prototype IFU.
	- Image slicer manufacture	HIGH	HIGH	Investigate diamond machining capabilities.
	- Image slicer alignment	HIGH	HIGH	Prototype and test mounts.
	- Pupil array manufacture	HIGH	HIGH	Investigate diamond machining capabilities.
	- Pupil array alignment	HIGH	MEDIUM	Align prototype mirror array.
	- Field array manufacture	HIGH	HIGH	Investigate diamond machining capabilities.
	- Field array alignment	HIGH	MEDIUM	Align prototype mirror array.
	- Optical blanks	MEDIUM	MEDIUM	Use common materials, f < 100 mm.
	- Optical alignment	MEDIUM	MEDIUM	Develop optical alignment strategy.
	- Baffling	MEDIUM	MEDIUM	Model cold stop region and baffles with OptiCAD.
	- Scattered light	MEDIUM	MEDIUM	Use silica mirrors and hand polishing where possible.
3	Mechanical
	- Spectrograph flexure	MEDIUM	MEDIUM	Detailed design followed by empirical adjustment.
	- Spectrograph alignment	MEDIUM	MEDIUM	Align spectrograph on setup CWS plate before installing in cryostat.
	- Cooler vibration	MEDIUM	MEDIUM	Monitor NIRI performance.
4	OIWFS
	- Setting accuracy	MEDIUM	HIGH	Monitor NIRI performance.
5	Detector
	- Delivery time	HIGH	HIGH	Place order early.
	- High dark current	HIGH	HIGH	Monitor Rockwell HgCdTe/CdZnTe development.
	- Excessive read noise	MEDIUM	MEDIUM	Implement linear fitting readout with SDSU-2 controller.
	- Excessive remnance	MEDIUM	MEDIUM	Monitor Rockwell HgCdTe/CdZnTe development.
	- Temperature stability	MEDIUM	HIGH	Build detector test dewar.

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